A new study by scientists from Ireland, Britain and Germany finds that the Atlantic Meridional Overturning Circulation (AMOC), also known as the Gulf Stream System, has never been as weak in the last millennium as it has in the last decades. They found consistent evidence that its slowdown is likely linked to human-caused climate change.
The AMOC is relevant for weather patterns in Europe and regional sea-levels in the US. “The Gulf Stream System works like a giant conveyor belt, carrying warm surface water from the equator up north, and sending cold, low-salinity deep water back down south. It moves nearly 20 million cubic meters of water per second, almost a hundred times the Amazon flow,” explains Stefan Rahmstorf from the Potsdam Institute for Climate Impact Research PIK, initiator of the study published in Nature Geoscience.
“For the first time, we have combined a range of previous studies and found they provide a consistent picture of the AMOC evolution over the past 1600 years,” says Rahmstorf. “The study results suggest that it has been relatively stable until the late 19th century. With the end of the little ice age in about 1850, the ocean currents began to decline, with a second, more drastic decline following since the mid-20th century.”
Since ongoing direct AMOC measurements only began in 2004, the researchers used proxy data to find out more about the long-term perspective of its decline. Proxy data, as witnesses of the past, consist of information gathered from natural environmental archives such as tree rings, ice cores, ocean sediments, and corals, as well as from historical data, for instance from ship logs.
Why is the AMOC slowing down?
An AMOC slowdown has long been predicted by climate models as a response to global warming caused by greenhouse gases – according to a number of studies, this is likely the reason for the observed weakening.
The Atlantic overturning is driven by what the scientists call deep convection, triggered by the differences in the density of the ocean water: Warm and salty water moves from the south to the north where it cools down and thus gets denser. When it is heavy enough the water sinks to deeper ocean layers and flows back to the south.
Global warming disturbs this mechanism: Increased rainfall and enhanced melting of the Greenland Ice Sheet add fresh water to the surface ocean. This reduces the salinity and thus the density of the water, inhibiting the sinking and thus weakening the flow of the AMOC.
Its weakening has also been linked to a unique substantial cooling of the northern Atlantic over the past hundred years. This so-called ‘cold blob’ was predicted by climate models as a result of a weakening AMOC, which transports less heat into this region.
Consequences of the AMOC slowdown
Levke Caesar, part of the Irish Climate Analysis and Research Unit at Maynooth University and guest scientist at PIK, explains: “The northward surface flow of the AMOC leads to a deflection of water masses to the right, away from the US east coast. This is due to Earth’s rotation that diverts moving objects such as currents to the right in the northern hemisphere and to the left in the southern hemisphere. As the current slows down, this effect weakens and more water can pile up at the US east coast, leading to an enhanced sea level rise.”
In Europe, a further slowdown of the AMOC could imply more extreme weather events like a change of the winter storm track coming off the Atlantic, possibly intensifying them. Other studies found possible consequences being extreme heat waves or a decrease in summer rainfall. Exactly what the further consequences are is the subject of current research; scientists also aim to resolve which components and pathways of the AMOC have changed how and for what reasons.
“If we continue to drive global warming, the Gulf Stream System will weaken further – by 34 to 45 percent by 2100 according to the latest generation of climate models,“ concludes Rahmstorf. “This could bring us dangerously close to the tipping point at which the flow becomes unstable.”
Image credit: GD Taber, flickr/Creative Commons